pyrometers - seoul national university

Review -

Pyrometers

Review -

PyrometersManually operated / Automatic

Sim Soo Young

Previously on “Pyrometers” by (1/4)

• Simplest & oldest non-contact way of estimating the temperature of a radiating body

by observing its color

• radiation thermometer (e.g. infrared thermometer)

• radiation wavelengths: visible & infrared radiation bands

(0.4 ~ 20 μm)

• Simplest & oldest non-contact way of estimating the temperature of a radiating body

by observing its color

• radiation thermometer (e.g. infrared thermometer)

• radiation wavelengths: visible & infrared radiation bands

(0.4 ~ 20 μm)


• Classification of pyrometers by wavelength and operating method


• Manually operated pyrometers

- operator’s eye acts as a comparator

1) Disappearing filament pyrometers

2) Two-color pyrometers (= Ratio pyrometers)

• Manually operated pyrometers

- operator’s eye acts as a comparator

1) Disappearing filament pyrometers

2) Two-color pyrometers (= Ratio pyrometers)


• Automatic pyrometers

- optical system concentrating the radiation on radiation detector

- radiation detector which may be either a thermal or a photoelectric sensor

- signal converter, conditioning the detector output signal before being displayed

- measuring part, which may have an additional analogue or digital output

1) Total radiation pyrometers 2) Photoelectric pyrometers

3) Two-wavelength pyrometers 4) Multi-wavelength pyrometers

• Automatic pyrometers

- optical system concentrating the radiation on radiation detector

- radiation detector which may be either a thermal or a photoelectric sensor

- signal converter, conditioning the detector output signal before being displayed

- measuring part, which may have an additional analogue or digital output

1) Total radiation pyrometers 2) Photoelectric pyrometers

3) Two-wavelength pyrometers 4) Multi-wavelength pyrometers

Manually Operated PyrometersManually Operated Pyrometers

1. Disappearing Filament Pyrometers

• History

The first pyrometer by the potter Josiah Wedgwood (1780s)

Modern pyrometers became available when the first disappearing filamentpyrometer was built by L.Holborn and F.Kurlbaum. (1901)

• History

The first pyrometer by the potter Josiah Wedgwood (1780s)

Modern pyrometers became available when the first disappearing filamentpyrometer was built by L.Holborn and F.Kurlbaum. (1901)


• Principle of operation

The brightness of a lamp filament is changed by adjusting the lamp currentuntil the filament disappears against the background of the target.

Eye of the observer is the detector. à lower limit of temperature range ≈ 700˚C

1. Disappearing Filament Pyrometers – Red filter

• Red filter (Scholl RG2, Jena 4512)

Comparison occurs at one wavelength è The effective wavelength, λe =0.65μm

λe is nearly constant at all measured temperatures. è (1300Kà3600K, Δ: 0.003μm)

Comparison occurs at one color è subjective estimation of color cannot influencethe measurement results.


• Red filter

Steepness of the curve W0,λ=0.65 = f(T) is greater than that of W0 = f(T).

The spectral radiance difference at λ=0.65 is greater than that of total radiance.

L0,λ = CW0,λ (Weichert, 1976) (L: spectral radiance, C: constant, W: radiant intensity)

• Reasons for the application of a red filter

1) Comparison takes place only at one wavelength è eliminating the influence ofsubjective color estimation by different observers

2) At λe = 0.65μm, the lowest possible temperature can be measured.

3) At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.

4) Easy to produce good filters of λe = 0.65μm which are stable in time

5) The smallest color changes as a function of wavelength are observed.


• Reasons for the application of a red filter

1) Comparison takes place only at one wavelength è eliminating the influence ofsubjective color estimation by different observers

2) At λe = 0.65μm, the lowest possible temperature can be measured.

3) At λe = 0.65μm, pyrometer sensitivity is higher than for the total radiation.

4) Easy to produce good filters of λe = 0.65μm which are stable in time

5) The smallest color changes as a function of wavelength are observed.

1. Disappearing Filament Pyrometers - Indicator

• Scale defining equation for black bodies

(Wien’s law)

(Weichert, 1976)

(L: spectral radiance, W: radiant intensity, λ: wavelength, Tt: true temperature)

• Black body: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)

(Vλ: relative spectral sensitivity of a standard human eye)

• Filament: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)


(εf λ : spectral emissivity of the filament)

• Filament: physiological feeling of brightness through a red filter of spectral transmissivity (τλ)

• The brightness of the filament and of the target are equal & λ=λe


(Tf: filament temperature, I: lamp current)

• Direct calibration of the ammeter of the pyrometer in temperature units.è Radiance temperature of a target at the wavelength, λe

• Scale divisions of the temperature scale is not linear.


• For a non-black body…. How to calibrate? (Equation)

(ελe: non-black body of spectral emissivity, Ti: pyrometer readings of black body)

(black body)C2: 1.4388 x 10-9 m•Kλe: 0.65μm

(non-black body)


• For a non-black body…. How to calibrate? (Diagram)

1. Disappearing Filament Pyrometers – Grey filter

• The tungsten filament can only be used up to 1400˚C.

• Dark deposit on the glass è changing the lamp characteristic

• To extend the pyrometer measurement up to 2000˚C a grey filter is used.

• A grey filter: reduce the target radiance.

1. Disappearing Filament Pyrometers – Grey filter

• For a black body

(w/o grey filter)

(w/ grey filter, τ'λe: spectral transmissivity)

Denote:

A: Radiance reducing factor of the grey filter

With grey filter, it’s possible to calibrate the pyrometer above the maximumfilament temperature.

1. Disappearing Filament Pyrometers – Applications

• Typical applications

- Comparison measurement in calibration of total radiation pyrometers

- Temperature measurement of small size targets ( 0.1 mm)

- Temperature measurement in research laboratories

- Comparison measurement of temperature of non-black bodies

- Measurement of temperature uniformity inside furnace chambers

• The application of disappearing filament pyrometers in industry has become less frequent.

• Typical applications

- Comparison measurement in calibration of total radiation pyrometers

- Temperature measurement of small size targets ( 0.1 mm)

- Temperature measurement in research laboratories

- Comparison measurement of temperature of non-black bodies

- Measurement of temperature uniformity inside furnace chambers

• The application of disappearing filament pyrometers in industry has become less frequent.

2. Two-Color Pyrometers

• General information

= Ratio pyrometer

The ratio of spectral radiances at 2 wavelengths is estimated by the human eye

The observer adjusts the filter position so that the target appears to be grey.

Target temperature increases è the percentage of green ↑, red ↓

• General information

= Ratio pyrometer

The ratio of spectral radiances at 2 wavelengths is estimated by the human eye

The observer adjusts the filter position so that the target appears to be grey.

Target temperature increases è the percentage of green ↑, red ↓

Color temperature

Measurement range: 1200 to 2000˚C

Error: ± 20 to 30 ˚C


• Scale defining equation (For a black body and a grey body)

(W: Spectral radiant intensity, ελ1 : emissity)

(For grey body, ελ is constant.)

The ratio of spectral radiant intensities is a function of the temperature

Target temperature increases è the ratio of Wλ1/ Wλ2 decreases.


RED

GREEN

Reduction in the precision of temperature measurement with increasing target temperature

The upper limit of the pyrometer: 2200˚C The lower limit of the pyrometer: 700 ˚C

GREEN


• Scale defining equation (For a non-grey body)

Non-black and non-grey bodies : selectively radiating bodiesè wavelength dependence of their spectral emissivityè ελ1 ≠ ελ2

Manually operated two-color pyrometers are now being replaced by automatic ones.

Automatic PyrometersAutomatic Pyrometers

1. Optical Systems

• To reach a sufficiently high measurement precision…è lenses, light-guides, or mirrors

• Lenses should be….

1) High transmission factor over a wide wavelength range2) High mechanical strength3) High working temperature4) Good resistance to atmospheric and chemical influences5) Good resistance to abrasion6) Good resistance to rapid temperature variations

1. Optical Systems – lenses or mirrors

• In pyrometry, the upper cut-off wavelength of incident radiation is important.è determines the lowest temperature which the pyrometer can measure.

• Quartz- High mechanical and chemical resistance- Withstand rapid temperature variations

• KRS-5- Most commonly used material for lensesof low temperature pyrometers

- starting from -50˚C

• Silicon- Sometimes replaces KRS-5 - The Ardometer pyrometer(Siemens AG)

• Quartz- High mechanical and chemical resistance- Withstand rapid temperature variations

• KRS-5- Most commonly used material for lensesof low temperature pyrometers

- starting from -50˚C

• Silicon- Sometimes replaces KRS-5 - The Ardometer pyrometer(Siemens AG)

• Mirrors: the lowest measured temperatures, where no lenses may be applied .• Metals with high reflection factor (e.g. Gold)

1. Optical Systems – light guides

• When the objects are too small• When pyrometer would be endangered by excessive temperatures

èLight guides replace lenses.

• Absorption along the rod, imperfect reflection from the rod walls andreflection losses è some of the transmitted energy is lost.

• the angle of incidence > the critical angle

• Materials: Artificial sapphire(Al2O3) or quartz(SiO2)• Solid rod or flexible stranded fibreoptic cable of thin fibres

• When the objects are too small• When pyrometer would be endangered by excessive temperatures

èLight guides replace lenses.

• Absorption along the rod, imperfect reflection from the rod walls andreflection losses è some of the transmitted energy is lost.

• the angle of incidence > the critical angle

• Materials: Artificial sapphire(Al2O3) or quartz(SiO2)• Solid rod or flexible stranded fibreoptic cable of thin fibres

2. Radiation Detectors – thermal radiation detector

• Used in <total radiation pyrometers>• Detector is heated by incident radiation.

• Detector should be….

1) High sensitivity (output signal / incident radiation power)2) Time stable properties3) High resistance to shocks and vibrations4) Low thermal inertia5) Output signal independent of the pyrometer position6) High output signal-to-noise ratio7) High emissivity8) Sensitivity independent of wavelength

• Thermopile:most commonly used, the reference junctions at the pyrometer housing temperature• Thermistor and metal bolometers:used in AC bridge circuits è easy amplification of the output signals• Pyroelectric detectors:

low temperature radiation pyrometers, high sensitivity but complicated construction of pyrometer

• Used in <total radiation pyrometers>• Detector is heated by incident radiation.

• Detector should be….

1) High sensitivity (output signal / incident radiation power)2) Time stable properties3) High resistance to shocks and vibrations4) Low thermal inertia5) Output signal independent of the pyrometer position6) High output signal-to-noise ratio7) High emissivity8) Sensitivity independent of wavelength

• Thermopile:most commonly used, the reference junctions at the pyrometer housing temperature• Thermistor and metal bolometers:used in AC bridge circuits è easy amplification of the output signals• Pyroelectric detectors:

low temperature radiation pyrometers, high sensitivity but complicated construction of pyrometer

2. Radiation Detectors – thermal radiation detector

Thermopile Thermistor Pyroelectricdetector

2. Radiation Detectors – photoelectric detector

• Used in <two/multi-wavelength pyrometers>

• Photoconductors:incident radiation à captured incident photons à photoelectrons à current

• Photodiodes:conductivity s proportional to the intensity of the radiation

• Photovoltaic cells:generated voltage is a logarithmic function of the incident radiation

• Vacuum photocells:incident radiation à emission of electrons from a metallic photocathode in a vacuum glass


• Used in <two/multi-wavelength pyrometers>

• Photoconductors:incident radiation à captured incident photons à photoelectrons à current

• Photodiodes:conductivity s proportional to the intensity of the radiation


• Vacuum photocells:incident radiation à emission of electrons from a metallic photocathode in a vacuum glass



Photoconductors Photodiodes Photovoltaic cells

Vacuum photocells Photomultipliers


• Properties of detectors

PbS, CdS : Photoconductors

Si, Ge: Photodiodes

InSb: Photovoltaic cells

3. Total Radiation Pyrometers

• The temperature of a body is determined by the thermal radiation,which it emits over a large range of wavelengths.

• This radiation is concentrated onto a thermal radiation detector


• Scale defining equation (For a black body)

(Ap: Plate area)

(K2: Heat transfer coefficient by convection and conduction)

when the plate is in the thermal steady-state…

• Scale defining equation (For a non-black body)


simplify

• Equation • Diagram


• Influence of housing temperature

to make the readings independent of the housing temperature,the difference between Tp and TH should be equal.

• Influence of target distance

the whole field of view should be filled by the target area

• Influence of target distance

the whole field of view should be filled by the target area

• Extension of measurement range

with grey filter è weakening the radiant flux coming from the object

(Tt: reading of pyrometer with grey filter,T’t: measured temperature,

τ1: filter transmission factor)

4. Photoelectric Pyrometers

• Measurement of rapidly changing temperatures

Total radiation pyrometers: 1 ms ~ 15 msPhotoelectric pyrometers: 1 ~ 2 μs

Photoelectric PyrometerwithDirect radiant flux

Photoelectric PyrometerwithDirect radiant flux

Photoelectric PyrometerwithModulated radiant flux


• Scale defining equation (For a black body)

the output signal of the photoelectric radiation detectors is proportionalto the number of photons (N)

(Warnke, 1972)

(in a narrow temperature range)

(IT: output current, B: constant, T: black body temperature,n: 5~12)

• Scale defining equation (For a non-black body)


(Reynolds, 1961)

Band emissivity is never precisely known.Band emissivity is never precisely known.

in practical, (Worthing, 1941)

5. Two-Wavelength Pyrometers

• Automatic two-color pyrometers

• The eye of observer is replaced by a photoelectric detector.

Rotating disk

Light guideLight guideMirror

6. Multi-Wavelength Pyrometers

• Used to measure the temperature of non-grey bodies of low emissivity

• Good precision of the method in the temperature range up to about 1700˚C

• Splitting the incoming radiation : light guide systems, filters, and prisms

Thank you.Any questions?Thank you.Any questions?

pyrometers - seoul national university

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